Aphid Alert 2003, No. 12, September 5

Potato insect update for the Northern Great Plains, week ending September 5

Total aphid captures in Minnesota and North Dakota (453) during the past week were half that of the week pervious. Only one green peach aphid was captured. Species most abundantly represented in Minnesota and North Dakota trap captures were: turnip aphid (220, most were from Hoople or Walhalla), corn leaf aphid (155), and sunflower aphid (46). Other potential virus vectors captured included: bird cherry-oat aphid (6), buckthorn aphid (5), soybean aphid (5), thistle aphid (5), and potato aphid (1).

Total aphid captures in Manitoba (755) likewise were half that of the week previous, but aphid captures per trap were for the second consecutive week higher than in the U.S. Eleven green peach aphid were captured in Manitoba. Species most abundantly represented in the Manitoba trap captures were: buckthorn aphid (576, =75% of total captures), bird cherry-oat aphid (126), potato aphid (9) and other (38).

The cartoon (below) shows cumulative capture of winged green peach aphid as mean number per trap (Minnesota and North Dakota data only) for the years 1992-1994, and from 1998 to the present. The gold line represents cumulative mean captures per trap to date for 2003. In the years we have operated the Aphid Alert trapping network, there were two with exceptionally high green peach abundance (1998 and 1999) and two with very few green peach aphids (1993 and 2001). Green peach aphid numbers have been relatively low this summer. This year, there appears to have been greater risk of mosaic spread than of leafroll virus because of the abundant inoculum present in the crop and because the former can be spread by other aphids in addition to green peach aphid.

Note that the data is plotted in this cartoon on a logarithmic scale.

The cartoon (below) shows cumulative capture of winged bird cherry-oat aphid as mean number per trap (Minnesota and North Dakota data only) for the years 1992-1994, and from 1998 to the present. The gold line represents cumulative mean captures per trap to date for 2003. For the first half of the 2003 growing season, bird cherry-oat captures were at the highest level we had recorded in the nine years we have operated the Aphid Alert network. In late summer with the onset of hot dry weather, bird cherry-oat aphid captures slowed and the cumulative captures ended below that of 2002. Our previous data suggests that cherry-oat aphid and green peach aphid are the principal vectors of PVY in our region. State seed potato inspectors have seen a lot of PVY (mosaic) in the field this summer. The prospects for this year's winter tests do not look good with respect to PVY.

Late Blight Situation

Late blight continues to be a potential threat in northeastern ND. New sightings show that late blight is still active and the recent rain and storms have made conditions in the region favorable for late blight spread and infection.

Growers in the northeastern ND should maintain regular fungicide program for the remainder of the season. Late blight can travel long distances in storms systems, as much as 50 miles, so growers in eastern ND and western MN should watch their fields carefully for the occurrence of late blight. The time from exposure to visible disease is 7-10 days. Because of the threat of late season infection, growers should continue their fungicide program through harvest. Late season late blight infections may go unnoticed and can result in tuber infection. Infected tubers can decay in storage from secondary tuber rot caused by Erwinia bacteria and cause severe loses in storage. Late season applications of Gavel, Omega or chlorothalonil provide the best tuber protection. To minimize late blight in storage be sure vines are dead; avoid wet harvest conditions to prevent free water in storage.

Remember to get a confirmation of late blight in the field if there is a potential claim with crop insurance for storage decay due to late blight.

In Manitoba (ACE, Sept 4) seven day severity values have remained largely unchanged since Tuesday, as rainfall and relative humidity levels have been low in the past few days. Sunny and above normal temperatures are forecast for the next 3-4 days.

Subscriber Alert

This is the twelfth issue of Aphid Alert 2003. This newsletter is intended to alert seed potato producers in the Northern Great Plains of the U.S. and Canada to flight activity by aphid species known to be potential vectors of potato viruses. These reports are posted weekly on the WWW and sent by e-mail to subscribers and by surface mail to all Minnesota and North Dakota seed potato growers.